The present invention relates to a method of surface-treating internal components of a nuclear reactor, and a nuclear power plant and, more particularly, to techniques for inhibiting the corrosion of internal components of a nuclear reactor exposed to reactor water.
Injection of hydrogen into reactor water is used prevalently as one of many methods to cope with mitigate IGSCC (intergranular stress-corrosion cracking) in the internal components of a reactor or pipes in a nuclear power plant. A large amount of hydrogen needs to be injected into reactor water to reduce the IGSCC potential below a critical potential. The critical potential of a stainless steel forming most of the structural components of a reactor, such as a reactor pressure vessel, is in the range of xe2x88x92230 to xe2x88x92300 mVvsSHE. Increase in the amount of hydrogen injected into reactor water involves increase in the amount of radioactive nitrogen that enters a main steam system and increases main steam dose rate. Hereinafter, structural components of a nuclear reactor will be referred to as reactor internal components.
This problem can be solved by noble metal injection, i.e., injection of a solution containing a noble metal into reactor water, mentioned in Japanese Pat. No. 2818943 and Japanese Patent Laid-open No. Hei 7-198893. The noble metal injected into reactor water deposits on the surfaces of the nuclear internal components including the components of the reactor pressure vessel and the reactor. An anodic reaction causing the oxidation of hydrogen is promoted by the catalytic effect of the noble metal, such as Pt, Rh or Pd. In a state where the noble metal is injected into reactor water, as compared with a state where any noble metal is not injected into reactor water, the electrochemical corrosion potential of the nuclear internal components can be reduced below the. critical potential by injecting a less amount of hydrogen into reactor water. However, noble metal injection needs at least hydrogen injection while the nuclear power plant is in normal operation. Although a method that injects an organometallic compound into reactor water does not need additional hydrogen injection while reactor water contains organic substances, the methods needs the reduction of electrochemical corrosion potential by hydrogen injection to keep the electrochemical corrosion potential on a low level.
It is an object of the present invention to provide a method of surface-treating internal structural components of a nuclear power plant and a nuclear power plant, capable of reducing the electrochemical corrosion potential (ECP) of the internal structural components of the nuclear power plant regardless of whether hydrogen is injected into reactor water.
According to a first aspect of the present invention, a method of surface-treating a reactor internal component of a nuclear power plant wets the surfaces of the reactor internal components to be exposed to reactor water with an electroless plating solution containing an electrical insulating substance. In other words, this method brings the surface of the reactor internal components into contact with the reactor water with the electroless plating solution. The electrical insulating substance has a resistivity about 105 (100000) times that of the reactor internal components or above. When the surfaces of the reactor internal components are wetted with the electroless plating solution, a metal film containing the electrical insulating substance is formed on the surfaces of the reactor internal components exposed to reactor water. Consequently, the electrochemical corrosion potential of the reactor internal components can be reduced regardless of whether hydrogen is injected into reactor water. The metal film containing the electrical insulating substance suppresses the growth of an oxide film on the surfaces of the reactor internal components exposed to reactor water by wet corrosion. Thus, the metal film suppresses increase in the surface dose rate of the reactor internal components due to increase in the inclusion of radioactive ions contained in reactor water in the oxide film as the oxide film grows due to wet corrosion.
According to a second aspect of the present invention, wets the surfaces of reactor internal components exposed to reactor water with an electroless plating solution containing an oxide having a lattice oxygen anion diffusing property. When the surfaces of the reactor internal components are wetted with the electroless plating solution, a metal film containing the oxide having a lattice anion diffusing property is formed on the surfaces of the reactor internal components. Since oxygen is supplied to the metal in contact with the oxide by the agency of the lattice anion diffusing property of the oxide, the metal not exposed to reactor water is oxidized by dry corrosion. Consequently, the electrochemical corrosion potential of the reactor internal components can be kept on a low level for a long period of time regardless of whether hydrogen is injected into reactor water. Since the oxide film formed by dry oxidation suppresses the growth of an oxide film on the surfaces of the reactor internal components exposed to reactor water due to wet oxidation, increase in the surface dose of the-reactor internal components can be suppressed.
According to a third aspect of the present invention, the surfaces of reactor internal components exposed to reactor water are wetted with an electroless plating solution containing at least either an electrical insulating substance or an oxide having a lattice oxygen anion diffusing property (hereinafter referred to as xe2x80x9celectrical insulating substance or the likexe2x80x9d) after the chemical decontamination of the reactor internal component. Since a metal film containing an electrical insulating substance or the like is formed on the surfaces of the reactor internal components exposed to reactor water, the electrochemical corrosion potential of the reactor internal components can be reduced regardless of whether hydrogen is injected into reactor water, and increase in the surface dose rate of the reactor internal components can be suppressed. Since corrosion products deposited on the surfaces of the reactor internal components can be removed by chemical decontamination, the metal film containing the electrical insulating substance or the like can be more uniformly formed.
According to a fourth aspect of the present invention, an electroless plating solution supply system having an injection nozzle for injecting an electroless plating solution containing at least an electrical insulating substance or the like is disposed between a reactor vessel and a reactor vessel flange, and the electroless plating solution supply system injects the electroless plating solution into the reactor pressure vessel.
According to a fifth aspect of the present invention, a metal film containing an electrical insulating substance or the like is formed on at least either a surface of a reactor vessel exposed to reactor water or surfaces of reactor internal components exposed to reactor water.